Photovoltaic cells have been developed which generate electrical energy directly from sunlight. Typically, such cells have been fabricated from semiconductor materials containing a rectifying junction, such as a p-n junction or a Schottky barrier.
A number of problems have been encountered, however, in attempts to produce photovoltaic cells which would have wide acceptance for use in producing energy from sunlight. One of the problems has been the cost of producing such cells, which has heretofore been relatively high. In fact, the cost of photovoltaic cells has generally been considered to be severely limiting except in applications where cost is not a controlling factor, such as in space applications or generation of power in remote areas.
The high cost of producing photovoltaic cells is due in large measure to the requirement for near crystal perfection and to the elaborate procedures involved in semiconductor wafer preparation. In regard to crystal perfection, it has almost universally been widely believed that only single crystal semiconductor materials could produce reasonable cell efficiencies. Polycrystalline semiconductor materials, on the other hand, have generally been unacceptable because of the degradation of the photovoltaic response caused in part by the shorting effect of grain boundaries (intersections between individual crystallites).
Because of the problems encountered with polycrystalline materials, they have not heretofore found wide acceptance for use in photovoltaic cells. Furthermore, those few photovoltaic cells which have employed polycrystalline materials have heretofore had much lower cell efficiencies than corresponding cells based upon single crystal semiconductor materials. Thus, although polycrystalline materials can be produced relatively inexpensively compared to single crystal materials, they have not been employed to any great extent in photovoltaic cells and, even when employed, have produced unsatisfactory photovoltaic cell efficiencies.
One solution to the problem posed by grain boundaries or other defects in polycrystalline semiconductor materials is the passivation process taught in U.S. Pat. No. 4,197,141 by Bozler and Fan. In this prior art process, passivation is generally accomplished by selective deposition of insulating caps at the points where the grain boundaries intersect a surface of the semiconductor material. The process entails employing the semiconductor material as an electrode in an electrolytic cell and passing electrical current along the grain boundaries. This produces anodization or selective deposition of an oxide cap at the surface intersection of the grain boundaries.
While the process of U.S. Pat. No. 4,197,141 represented a significant advance in the state-of-the-art at the time the invention was made, nevertheless, it only corrected the detrimental effects of grain boundaries or other dislocation defects at the surface between the contact bars or fingers and the top layer of the rectifying junction, or in the words of the patent, at the surface intersection of the grain boundaries. The process did not compensate the shorting effects caused by grain boundaries within the layer.
Consequently, a need exists for a simple, inexpensive, and more effective way of passivating grain boundaries or dislocation defects within one or more layers of a polycrystalline or single crystal semiconductors in order to compensate for the detrimental shorting effects of such grain boundaries or dislocations.